Reducing the feedback overhead during crosstalk precoder initialization

ABSTRACT

An apparatus comprising a receiver coupled to a digital subscriber line (DSL) between an exchange site and a customer premise equipment (CPE) and configured to send a feedback error message to train a precoder coupled to the exchange site, wherein the feedback error message comprises a plurality of error components and an indication of a quantity of bits per error component, a quantization accuracy per error component, or both. Included is a method comprising sending an error feedback message to a DSL crosstalk precoder to train the crosstalk precoder, wherein the error feedback message comprises an error vector and a quantization scaling factor of the error vector.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 61/148,827 filed Jan. 30, 2009 by Raphael JeanCendrillon, et al. and entitled, “Methods for Reducing Feedback OverheadDuring Crosstalk Precoder Initialization,” and U.S. Provisional PatentApplication No. 61/148,887 filed Jan. 30, 2009 by Raphael JeanCendrillon, et al. and entitled, “Methods and Systems for Reducing theFeedback Overhead During Crosstalk Precoder Initialization,” both whichare incorporated herein by reference as if reproduced in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Digital subscriber line (DSL) technologies can provide large bandwidthfor digital communications over existing subscriber lines. Whentransmitting data over the subscriber lines, crosstalk interference canoccur between the transmitted signals over adjacent twisted-pair phonelines, for example in a same or nearby bundle of lines. Crosstalk limitsthe performance of some DSL technologies, such as very high bit rate DSL2 (VDSL2). The crosstalk in the subscriber lines can be eliminated orreduced using a crosstalk precoder, such as in a modem. The precoder maybe used to modify and transmit signals from an exchange site downstreamto a plurality of customer premise equipments (CPEs). The signals may bepre-distorted in a determined manner such that the pre-distortion in thesignals and the crosstalk in the lines cancel out. Consequently,non-distorted signals that are substantially free of crosstalk may bereceived at the other end.

The precoder is trained or initialized using feedback signals from theCPEs, which may indicate the errors in the received signals at the CPEs.A sequence of pilot symbols are transmitted downstream to a VDSLtransceiver remote unit (VTU-R) at the CPE, which returns correspondingerror feedback signals to a VDSL transceiver office unit (VTU-O) at theexchange. The error feedback signals are used to train the precoder toadjust the pre-distorted signals until reaching convergence. The errorfeedback signals are provided from the CPEs to the exchange via a backchannel and typically require a substantial data rate, e.g. for aplurality of subscriber lines. If the data rate cannot be met by thenetwork standards, the feedback is provided to the precoder at a lowerrate, such as using a subset of the tones in the pilot symbols in thesubscriber lines. Using a subset of the tones to transmit a feedbacksignal may increase the initialization time of the precoder, lead toslower convergence of the precoder output, and reduce performance.

In some systems, to reduce the initialization time of the precoder, asampling of the error feedback signals may be provided, e.g. using fewerfrequencies in the error's frequency range. For example, the errorfeedback signal from a CPE may correspond to every n-th sub-carriersignal from a plurality of N sub-carriers, where N is the quantity ofsub-carriers. The remaining portion of the signal, e.g. corresponding tothe remaining frequencies or sub-carriers, may be interpolated from thereceived sampled feedback signal. However, using a sample of the errorfeedback signal to obtain a complete error feedback signal may reduceaccuracy and performance. In other systems, the error feedback may berepresented using fewer quantization bits, which may lead to slowererror convergence and reduce performance.

SUMMARY

In one embodiment, the disclosure includes an apparatus comprising areceiver coupled to a DSL between an exchange site and a CPE andconfigured to send a feedback error message to train a precoder coupledto the exchange site, wherein the feedback error message comprises aplurality of error components and an indication of a quantity of bitsper error component, a quantization accuracy per error component, orboth.

In another embodiment, the disclosure includes an apparatus comprisingat least one processor configured to implement a method comprisingdetermining a range of error for a plurality of error components of apilot signal, determining a quantity of bits for representing error suchthat a full error range is preserved, a quantization accuracy such thatthe full error range is represented by a fixed number of feedback bits,or both based on the range of error for the error components,transmitting an error feedback signal that comprises the errorcomponents and indicates the quantity of bits, the quantizationaccuracy, or both.

In yet another embodiment, the disclosure includes a method comprisingsending an error feedback message to a DSL crosstalk precoder to trainthe crosstalk precoder, wherein the error feedback message comprises anerror vector and a quantization scaling factor of the error vector.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of a DSL system.

FIG. 2 is a chart of an embodiment of a maximum error feedback during aprecoder training time.

FIG. 3 is a chart of another embodiment of a maximum error feedbackduring a precoder training time.

FIG. 4 is a chart of an embodiment of a convergence in quantity of errorfeedback bits.

FIG. 5 is a chart of an embodiment of a signal to noise ratio (SNR)improvement.

FIG. 6 is a chart of an embodiment of a data rate improvement.

FIG. 7 is a chart of another embodiment of a data rate improvement.

FIG. 8 is a schematic diagram of an embodiment of an error feedbackmessage.

FIG. 9 is a schematic diagram of another embodiment of an error feedbackmessage.

FIG. 10 is a schematic diagram of another embodiment of an errorfeedback message.

FIG. 11 is a flowchart of an embodiment of an error feedback signalingmethod.

FIG. 12 is a schematic diagram of one embodiment of a general-purposecomputer system.

DETAILED DESCRIPTION

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Disclosed herein is a system and method for reducing a crosstalkprecoder initialization time and improving performance in a DSL network.The precoder may be provided with an error feedback signal for eachline, which may be represented using a determined quantity of bits. In afirst embodiment, the quantity of bits may be determined based on adesired accuracy of the error feedback signal and the error range forthe line. In another embodiment, the quantity of bits may be fixed andthe quantization accuracy of the error feedback signal may be variedbased on the error range for the line. Alternatively, both the quantityof bits and the quantization accuracy may be varied depending on theerror range. During the initialization or training time of the crosstalkprecoder, the range of error of the feedback signal may decrease and theoutput of the crosstalk precoder may decrease. As the output of theprecoder decreases, the error feedback signal may be represented usingfewer bits without substantially reducing accuracy or may be representedusing higher quantization accuracy without substantially increasing thenumber of bits. Alternatively, the error feedback signal may berepresented using both fewer bits and higher quantization accuracy. Assuch, the crosstalk precoder may be trained using lower data rateswithout sacrificing the convergence speed of the precoder and reducingsystem performance.

FIG. 1 illustrates one embodiment of a DSL system 100. The DSL system100 may be a VDSL or VDSL2 system, an ADSL or ADSL2 system, or any otherDSL system. The DSL system 100 may comprise an Exchange 102 and aplurality of customer premise equipments (CPEs) 104, which may becoupled to the Exchange 102 via a plurality of subscriber lines 106. Atleast some of the subscriber lines 106 may be bundled in a binder 107.The DSL system 100 may also comprise a crosstalk precoder 108, which maybe coupled to the subscriber lines 106 and positioned between theExchange 102 and the CPEs 104. Additionally, the DSL system 100 mayoptionally comprise a network management system (NMS) 110 and a publicswitched telephone network (PSTN) 112, both of which may be coupled tothe Exchange 102. In other embodiments, the DSL system 100 may bemodified to include splitters, filters, management entities, and variousother hardware, software, and functionality.

The NMS 110 may be a network management infrastructure that processesdata exchanged with the Exchange 102 and may be coupled to one or morebroadband networks, such as the Internet. The PSTN 112 may be a networkthat generates, processes, and receives voice or other voice-bandsignals. In an embodiment, the Exchange 102 may be a server located at acentral office and may comprise switches and/or splitters, which maycouple the NMS 110, the PSTN 112, and the subscriber lines 106. Forinstance, the splitter may be a 2:1 coupler that forwards data signalsreceived from the subscriber lines 106 to the NMS 110 and the PSTN 112,and forwards data signals received from the NMS 110 and the PSTN 112 tothe subscriber lines 106. Further, the splitter may optionally compriseone or more filters to help direct data signals between the NMS 110, thePSTN 112, and the subscriber lines 106. Additionally, the Exchange 102may comprise at least one DSL transmitter/receiver (transceiver), e.g.VTU-O, which may exchange signals between the NMS 110, the PSTN 112, andthe subscriber lines 106. The signals may be received and transmittedusing the DSL transceiver, such as a modem.

In an embodiment, the DSL transceiver may comprise a forward errorcorrection (FEC) codeword generator that generates FEC data. The DSLtransceiver may also comprise an interleaver that interleaves thetransmitted data across a plurality of tones in a pilot symbol (or syncsymbol). For instance, the DSL transceiver may use a discrete multi-tone(DMT) line code that allocates a plurality of bits for each sub-carrieror tone in each symbol. The DMT may be adjusted to various channelconditions that may occur at each end of a subscriber line. In anembodiment, the DSL transceiver of the Exchange 102 may be configured totransmit data at similar or different rates for each subscriber line106.

In an embodiment, the CPEs 104 may be located at the customer premises,where at least some of the CPEs 104 may be coupled to a telephone 114and/or a computer 116. The telephone 114 may be hardware, software,firmware, or combinations thereof that generates, processes, andreceives voice or other voice-band signals. The CPE 104 may comprise aswitch and/or a splitter, which may couple the subscriber lines 106 andthe telephone 114 and the computer 116. The CPE 104 may also comprise aDSL transceiver, e.g. VTU-R, to exchange data between the CPE 104 andthe Exchange 102 via the subscriber line 106. For instance, the splittermay be a 2:1 coupler that forwards data signals received from thesubscriber line 106 to the telephone 114 and the DSL transceiver, andforwards data signals received from the telephone 114 and the DSLtransceiver to the subscriber line 106. The splitter may optionallycomprise one or more filters to help direct data signals to and from thetelephone 114 and the DSL transceiver.

The DSL transceiver, e.g. a modem, in the CPE 104 may transmit andreceive signals through the subscriber lines 106. For instance, the DSLtransceiver may process the received signals to obtain the transmitteddata from the Exchange 102, and pass the received data to the telephone114, the computer 116, or both. The CPEs 104 may be coupled to theExchange 102 directly via the subscriber lines 106 and/or via thesubscriber lines 106. For example any of the CPEs 104 may be coupled toa subscriber line 106 from the Exchange 102. The CPEs 104 may access theNMS 110, the PSTN 112, and/or other coupled networks via the subscriberlines 106 deployed by the Exchange 102.

In an embodiment, the subscriber lines 106 may be telecommunicationspaths between the Exchange 102 and the CPE 104 and/or between thecrosstalk precoder 108 and the CPEs 104, and may comprise one or moretwisted-pairs of copper cable. Crosstalk interference may occur betweenthe tones or signals transported through the subscriber lines 106 thatare deployed by the Exchange 102, e.g. in the binder 107. The crosstalkinterference may be related to the power, frequency, and travel distanceof the transmitted signals and may limit the communications performancein the network. For instance, when the power spectral density (PSD) ofthe transmitted signals increase, e.g. over a range of frequencies, thecrosstalk between the adjacent subscriber lines 106 may increase andhence the data rates may decrease. The propagation of the signals in thedownstream direction from the Exchange 102 to the CPEs 104 may berepresented by:y=Hx+z,  (1)where y is a vector that represents the signals at the CPEs 104, H is amatrix that represents the crosstalk channels in the lines, x is avector that represents the signals from the Exchange 102, and z is avector that represents random errors or noise.

In an embodiment, the crosstalk precoder 108 may be configured to reduceor limit the crosstalk in the lines. The crosstalk precoder 108 maytransmit pre-distorted signals in the subscriber lines 108 to cancel orreduce crosstalk error in the lines. The crosstalk precoder 108 mayreceive a plurality of signals from the Exchange 102 (e.g. from aplurality of VTU-Os), add distortion to the signals, and thus transmit aplurality of corresponding pre-distorted signals to the CPEs 104 via thesubscriber lines 106. The pre-distorted signals may be configured basedon a plurality of error feedback signals from the CPEs 104. Forinstance, a plurality of VTU-Rs at the CPEs 104 may measure the errorsfor a plurality of received pilot symbols from the Exchange 102, andtransmit back to the Exchange 102 a plurality of corresponding errorfeedback signals. The VTU-Os at the Exchange 102 may receive the errorfeedback signals, use the signals to identify the crosstalk channels inthe lines, and initialize a precoding matrix for the crosstalk precoder108. The precoding matrix may be obtained based on an adaptivealgorithm, such as a least mean square (LMS) algorithm or a recursiveleast square (RLS) algorithm. The crosstalk precoder 108 may use theprecoding matrix to produce the pre-distorted signals for the lines.Cancelling the crosstalk using signal distortion may be represented by:

$\begin{matrix}\begin{matrix}{y = {{HPx} + z}} \\{{= {{{diag}\left\{ H \right\} x} + z}},}\end{matrix} & (2)\end{matrix}$where P=H⁻¹ diag{H} is a precoding matrix configured to cancel orsubstantially eliminate the crosstalk channels in the lines.

The process of sending the pilot symbols (e.g. to the VTU-Rs) andreceiving corresponding error feedback signals (at the VTU-Os) may berepeated over a period of time to improve the output of the crosstalkprecoder 108 and hence improve crosstalk cancelation. Such period oftime may be referred to as the training or initialization time of thecrosstalk precoder 108. For instance, during the initialization time, asequence of pilot symbols may be transmitted and accordingly a sequenceof error feedback signals may be received (e.g. for each subscriber line106) until the pre-distorted signals from the crosstalk precoder 108converge to a pattern or value.

A feedback channel, which may have a predetermined bandwidth, may beallocated to transport the error feedback signals from the CPEs 104 tothe Exchange 102 or to the crosstalk precoder 108. The error feedbacksignals may correspond to a plurality of pilot symbols, which may eachcomprise a plurality of tones. Each tone may be represented by aplurality of bits in the signal. The quantity of bits used may determinethe quantization accuracy of the error range, e.g. as measured by theVTU-Rs at the CPEs 104. The quantization accuracy may be such that thefull error range is represented by a fixed number of feedback bits.Additionally, the quantity of bits may determine the range of errorsthat may be measured. Typically, a substantially large bandwidth or datarate may be needed to provide accurate error feedback and minimize orlimit the initialization time of the crosstalk precoder 108.

For example, the error feedback signal may comprise about 48,000 bitsfor each pilot symbol that comprises about 3,000 tones, where the realcomponent for each tone may be represented by about 16 bits. The totalnumber of bits for each tone may comprise about eight bits for the realcomponent of the tone and about eight bits for the imaginary componentof the tone. Accordingly, if this error feedback signal is providedevery about 64.25 milliseconds (ms), e.g. during the training time, thefeedback channel may require at least about 747 kilobits-per-second(kbps). Such data rate may exceed the bandwidth limitations in some DSLsystems. For example, in VDSL2, a feedback channel or a specialoperation channel (SOC) may support about 64 kbps, which may not beenough to transport about 48,000 bits for each pilot symbol. Typically,in this case, the error feedback signal may be provided using fewer bitsto reduce the data rate in the feedback channel, which may also increasethe training time and reduce performance, e.g. in terms of achievabledata rates. In an embodiment, to reduce the training time and improveperformance, the quantity of bits and/or the quantization accuracy inthe error feedback signals may be adjusted without substantially loosingaccuracy or increasing overhead, as described in detail below.

FIG. 2 illustrates an embodiment of a maximum error feedback 200 duringa precoder training time, which may be obtained at an Exchange or acrosstalk precoder. The maximum error feedback 200 may be represented bya curve 210. The curve 210 may comprise a plurality of received maximumerror feedback values e_(max)(i) for a plurality of transmitted pilotsymbols i during the precoder training time. The received error feedbacksignal may be represented in complex format as:E(k,i)=e _(x)(k,i)+j·e _(y)(k,i),  (3)where E(k,i) is the error feedback signal for the pilot symbol i and atone k of the pilot symbol, and e_(x)(k,i) and e_(y)(k,i) are the realand imaginary components, respectively, of the error feedback signal.The error feedback signal may be used to obtain a precoding matrix totrain the precoder using the LMS algorithm. The maximum error feedbacksignal value for the pilot symbol i may be obtained from the real andimaginary components of the maximum error feedback for all the tones inthe pilot symbol, such as:

$\begin{matrix}{{{e_{\max}(i)} = {\max\limits_{k}\left\{ {\max\left\{ {{{e_{x}\left( {k,i} \right)}{,}{e_{y}\left( {k,i} \right)}}} \right\}} \right\}}},} & (4)\end{matrix}$where max{ } indicates a function for selecting a maximum sample from aset. As shown in FIG. 2, the maximum error feedback value may decreaseas the quantity of pilot symbols increases, e.g. as the training timeincreases. Additionally, the maximum error feedback value may convergeto about a fixed value as the training time increases.

FIG. 3 illustrates another embodiment of a maximum error feedback 300during a precoder training time, which may be obtained at an Exchange ora crosstalk precoder. The maximum error feedback 300 may be representedby a curve 310. The curve 310 may indicate a received maximum errorfeedback value e_(max)(i) vs. the precoder training time (in seconds).Similar to FIG. 2, the maximum error feedback value in FIG. 3 is foundto decrease and converge as the training time increases. For example,e_(max)(i) may be equal to about 0.16 at about the first second oftraining time and may decrease and converge to about 0.02 at about thetenth second of training time.

Typically, when a fixed quantity of bits is used to transmit the errorfeedback signal, the pilot symbols that have smaller error range andsmaller maximum error feedback value may be represented with higheraccuracy. Since the maximum error feedback may decrease as the trainingtime increases (as shown in FIG. 2 and FIG. 3), the quantizationaccuracy of the feedback error signal may increase as the training timeincreases when a fixed quantity of bits is used. For example, when abouteight bits are used to transmit the error feedback signal, thequantization accuracy of the error feedback signal that has an errorrange between about −1 and about 1 may be equal to about 2⁻⁷ or about0.0078. In comparison, using about the same quantity of bits, thequantization accuracy of the error feedback signal that has an errorrange between about −0.25 and about 0.25 may be substantially increasedto about 2⁻⁹ or about 0.002.

If the quantity of quantization bits is reduced as the training timeincreases and the error range decreases, the quantization accuracy ofthe feedback error signal may remain about the same. For example, thequantization accuracy of the error feedback signal that has an errorrange between about −1 and about 1 may be equal to the quantizationaccuracy of the error feedback signal that has an error range betweenabout −0.25 and about 0.25 when the quantity of quantization bits isreduced from about eight bits to about six bits. Reducing the quantityof quantization bits as the training time increases and the output ofthe crosstalk precoder converges may reduce overhead and bandwidth ofthe feedback channel. Additionally, as the quantity of used bitsdecrease, the training time may decrease and performance may improve.

In an embodiment, a quantity of bits for representing error such that afull error range is preserved may be determined. The quantity of bitsthat may be used to represent the error feedback signal, N_(r)(i), maybe determined based on a desired quantization accuracy, d, for the tonesand the maximum error feedback in the pilot symbol e_(max)(i), such as:

$\begin{matrix}{{N_{r}(i)} = {{\log_{2}\left( \frac{2{e_{\max\;}(i)}}{d} \right)}.}} & (5)\end{matrix}$For instance, a VTU-R at the CPE may represent the error feedback signalfor each tone in the pilot symbol using the determined quantity of bitsN_(r)(i) and send this information, e.g. in a message, to a VTU-O at theExchange. The VTU-R may also indicate to the VTU-O the determinedquantity of quantization bits N_(r)(i) in the message.

FIG. 4 illustrates an embodiment of a convergence in quantity of errorfeedback bits 400 during a precoder training time. The error feedbackbits may be sent from a CPE to an Exchange or a crosstalk precoder. Theconvergence in quantity of error feedback bits 400 may be represented bya curve 410. The curve 410 may comprise a quantity of error feedbackbits, N_(r)(i), for each of the transmitted pilot symbols i during theprecoder training time. The quantity of error feedback bits mayrepresent a plurality of error feedback signals, e.g. as received by aVTU-O in the Exchange. The quantity of error feedback bits N_(r)(i) maybe determined based on a desired quantization accuracy, d, for the tonesand the maximum error feedback in the pilot symbol e_(max)(i), as shownin the equation above. Accordingly, the quantity of error feedback bitsN_(r)(i) may be proportional to the maximum error feedback in the pilotsymbol e_(max)(i). The maximum error feedback in the pilot symbole_(max)(i) may be equal to about the maximum error feedback in the pilotsymbol e_(max)(i) in FIG. 2.

In FIG. 4, the error feedback signal may be initially transmitted usingabout eight bits per error component and may have an error range betweenabout −1 and about 1. The quantization accuracy d of the initial errorfeedback signal may be equal to about 2⁻⁷ or about 0.0078. The quantityof error feedback bits N_(r)(i) may then decrease as the quantity ofpilot symbols increases, e.g. as the training time increases. Thequantity of error feedback bits N_(r)(i) may converge to about four pererror component as the training time increases. In FIG. 2, it was shownthat the maximum error feedback in the pilot symbol e_(max)(i) maydecrease and converge as the quantity of pilot symbols and the trainingtime increase. Consequently, since the quantity of error feedback bitsN_(r)(i) may be proportional to the maximum error feedback in the pilotsymbol e_(max)(i), the quantity of error feedback bits N_(r)(i) may alsodecrease and converge as the quantity of pilot symbols and the trainingtime increase, as shown in FIG. 4. The decrease in the quantity of errorfeedback bits N_(r)(i) may reduce the feedback data rate, increaseprecoder training time, and improve performance.

For example, at convergence, the quantity of total feedback bits in thetransmitted pilot symbols may be equal to about 4.3×10⁶. This may be areduction of about 71 percent in comparison to the case of training theprecoder using a fixed quantity of bits at about eight bits per errorcomponent, where the quantity of total feedback bits may be of about14.9×10⁶. Further, since the quantity of error feedback bits N_(r)(i) iscalculated without substantially changing the quantization accuracy d ofthe initial error feedback signal, the decrease in the quantity of errorfeedback bits N_(r)(i) may not add substantial overhead in terms ofaccuracy for crosstalk reduction.

FIG. 5 illustrates an embodiment of a SNR improvement 500 during aprecoder training time. The SNR improvement 500 is shown for a pluralityof pilot symbols received by an Exchange or a crosstalk precoder, e.g.as transmitted from the CPE. The SNR improvements 500 may be representedby a curve 510. The curve 510 may comprise a SNR value for each of thetransmitted pilot symbols i during the precoder training time. The pilotsymbols may be transmitted by adjusting the quantity of error feedbackbits, N_(r)(i), as shown in curve 410. As shown in FIG. 5, the SNR valuemay increase and converge as the quantity of pilot symbols and thetraining time increase. The curve 510 may be compared to another curve520, which may comprise the SNR values in an ideal precoder. The idealprecoder may eliminate the crosstalk in the line without substantialtraining time. As shown, the SNR value in the curve 510 may reach aboutthe same SNR value of the curve 520 at the convergence point, e.g. atabout 400 transmitted pilot symbols.

The curve 510 may also be compared to another curve 530, which maycomprise the SNR values for the transmitted pilot symbols i using abouteight bits per error component. The quantity of feedback bits in curve530 may be fixed for all the transmitted pilot symbols. This fixedquantity of bits may be equal to the initial quantity of bits per errorcomponent in curve 510 and to about twice the quantity of bits at theconvergence point of curve 510. The two curves 510 and 530 are found tooverlap and may comprise substantially about the same SNR values. Thismay indicate that reducing the quantity of error feedback bits, e.g.based on a desired quantization accuracy and the maximum error feedbackin the pilot symbols, may reduce system overhead without substantiallyreducing accuracy.

In another embodiment, the quantity of error feedback bits may be keptfixed during the training time and the quantization accuracy, e.g. pererror component, may be increased. As such, the quantization accuracy,d, may be adjusted based on the quantity of error feedback bits,N_(r)(i), and the maximum error feedback in the pilot symbols,e_(max)(i), such as:

$\begin{matrix}{{d(i)} = {\frac{2\;{e_{\max}(i)}}{2^{N_{r}}}.}} & (6)\end{matrix}$Adjusting the quantization accuracy during the training time may lead tosubsequently smaller feedback errors and thus promote faster convergencein the output of the crosstalk precoder. Consequently, this may lead toreducing the training time and improving performance. Further, since thequantity of error feedback bits may be kept constant, no increase inoverhead may be needed. For instance, a VTU-R at the CPE may representthe error feedback signal for each tone in the pilot symbol using thequantization accuracy d (i) and send this information, e.g. in amessage, to a VTU-O at the Exchange. The VTU-R may also indicate to theVTU-O the determined accuracy d(i) in the message.

In an embodiment, to adjust the quantization accuracy of the errorfeedback signals, an error vector (per error component) may be scaled toguarantee using substantially the full the error range. First, a scalingfactor S_(Q)(i) may be selected from a set of scaling factors, e.g. (1,2, . . . , 256). The scaling factor may be selected such thatS_(Q)(i)·e_(max)(i)≦1, e.g. for a quantization range between about −1and about 1, to avoid clipping the error feedback signal. The errorcomponents (e.g. real and imaginary components) may then be scaled bythe scaling factor, such as:ē _(x)(k,i)=S _(Q)(i)·e _(x)(k,i),  (7a)ē _(y)(k,i)=S _(Q)(i)·e _(y)(k,i),  (7b)where ē_(x) and ē_(y) are the scaled error vectors. The quantizationaccuracy may be increased by scaling the error vectors before digitizingor representing the error feedback signals in bits.

For instance, the error feedback signal may be represented based on aquantization format proposed by the International TelecommunicationUnion (ITU) Telecommunication Standardization Sector (ITU-T) documentC-91, which is incorporated herein by reference. Thus, the errorfeedback signal may be represented in complex format as:E _(x)(k,i)=max{−2^(N-1),min{S _(Q)(i)·e_(x)(k,i)·2^(N-1),2^(N-1)−1}},  (8a)E _(y)(k,i)=max{−2^(N-1),min{S _(Q)(i)·e_(y)(k,i)·2^(N-1),2^(N-1)−1}},  (8b)where E_(x)(k,i) and E_(y)(k,i) are the real and imaginary components,respectively, of the error feedback signal. In other embodiments, otherquantization formats may be used to adjust the error vectors and hencethe quantization accuracy of the error feedback signal. Accordingly, thescaling factor may be dynamically adjusted to fit substantially theerror range and increase the quantization accuracy as the training timeincreases and the quantity of quantization bits remains fixed. Forexample, using about eight quantization bits, the maximum error feedbackvalue may reach about 2⁻⁷ during a precoder training time and thescaling factor may be adjusted accordingly to about 2⁷, which may resultin a quantization accuracy of about 2⁻¹⁴. In comparison, about 15quantization bits may be needed to achieve the same quantizationaccuracy using conventional quantization, e.g. without scaling the errorvectors.

In an embodiment, the VTU-R at the CPE may indicate the used scalingfactor S_(Q)(i) to the VTU-O at the Exchange. The VTU-R may send anerror feedback message (e.g. R-ERROR_FEEDBACK) that comprises thescaling factor S_(Q)(i) to the VTU-O. The same scaling factor may beused for all the tones in a pilot symbol, and hence a single field inthe error feedback message may be needed to indicate the scaling factorS_(Q)(i). For instance, the error feedback message R-ERROR_FEEDBACK inTable 10-4 of the ITU-T standard G.vector, which is incorporated hereinby reference, may be modified to include a field “Quantization ScalingFactor” (e.g. Field #3), as shown below.

TABLE 1 A Modified Version of Table 10-4 of the ITU-T Standard G.vector.Field Name Format 1 Message Descriptor Message Code 2 Frequency Band ID1 byte 3 Quantization Scaling Factor 1 byte 4 Error Vector N_(bytes)bytes

When a LMS algorithm is used to obtain the precoding matrix and trainthe crosstalk precoder, the error in the pilot symbols and accordinglythe error feedback signals may be reduced in an asymptotic manner, e.g.may converge to a level or value. The convergence level may be dependenton the level of the quantization noise in the error feedback signal.Scaling the error vector may reduce the quantization noise and increasethe LMS step size, which may reduce the asymptotic error (or convergencelevel) and increase the quantization accuracy. Additionally, reducingthe LMS step size may increase the convergence rate and reduce theprecoder training time.

FIG. 6 illustrates an embodiment of a data rate improvement 600 during aprecoder training time. The data rate improvement 600 is shown forsimulated pilot symbols in a selected line. The selected line may bebonded with a plurality of other lines, including about 14 legacy linesand about 17 active vectored lines. Specifically, a second phase (e.g.R-P-VECTOR2) of crosstalk precoder initialization was simulated, wherethe precoder may learn to cancel crosstalk from the active lines intothe selected line. According to the ITU-T document C-140 (which isincorporated herein by reference), the second phase may be the longestphase of crosstalk precoder initialization and therefore maysubstantially determine the total initialization time for the precoder.

The data rate improvement 600 may be represented by a curve 610. Thecurve 610 may comprise a data rate value for each of the transmittedpilot symbols during the precoder training time. The pilot symbols maybe transmitted by adjusting the error vector and hence the accuracy ofthe error feedback signals. The error vector was adjusted by selecting ascaling factor value from the values 1, 2, 4, 8, 16, 32, 64, 128, and256. The scaling factor was limited to such set of values to simplifythe multiplication of the error vector by the scaling factor, e.g. usinga left shift operation. Some of the simulation parameters that were usedare shown in Table 2. The quantity of bits used to represent the pilotsymbols during the training time is fixed at about eight bits. As shownin FIG. 6, the data rate value may increase and converge as the trainingtime increases. The curve 610 may be compared to another curve 620,which may comprise the data rate values in an ideal precoder, e.g. whichmay cancel the crosstalk in the line without substantial training time.The data rate value in the curve 610 may reach about 140 megabits persecond (Mbps) after about eight seconds, which may be substantiallyclose to the data rate value of the curve 620.

TABLE 2 Parameter Value Loop type arrayed waveguide grating (AWG) 26Number of lines Total = 32; 14 legacy + 18 vectored (17 busy, 1 joining)Length of orthogonal sequence 32 (duration = 2 seconds) Loop length 300meters Symbol rate 4000 symbols/second Transmit power −60 decibel(dBm)/Hertz (Hz) Noise −135 dBm/Hz Bandplan 17a Far-end crosstalk (FEXT)model Alcatel-Lucent (NIPP-NAI 2008-010R1) Back channel Extended SOCchannel with 4 or 8 bits per complex error sample Coding gain 2 dB SNRmargin 6 dB Bit error rate (BER) 10⁻⁷ Valid scaling factors 1, 2, 4, 8,16, 32, 64, 128, 256

The curve 610 may also be compared to curves 630 and 640, which maycomprise the data rate values for the transmitted pilot symbols usingconventional quantization (e.g. with fixed quantization accuracy) andthe LMS algorithm. Specifically, the curve 630 was obtained using a LMSstep size μ equal to about 0.01, and the curve 640 was obtained using aLMS step size μ equal to about 0.02. The data rate value in the curve630 reaches about 140 Mbps after about 20 seconds. The improvement inthe training time of the curve 610 in comparison to the curve 630 may beequal to about 60 percent. Although the curve 640 may reach convergenceat about the same time as the curve 610, the data rate value in thecurve 640 at convergence may be about 131 Mbps, which is substantiallylower than the curve 610 that has a data rate value at about 140 Mbps.

Adjusting the LMS step size and using conventional quantization (e.g.fixed error scaling) may improve the training time of the precoder atthe expense of accuracy and achievable data rate, as shown in the curves630 and 640. This tradeoff between the training time and achievable datarate may be overcome by adjusting the quantization accuracy duringtraining time, as shown in the curve 610. Adjusting the scaling factorin the error feedback signals may guarantee that the error values occupysubstantially the entire error range, which may improve the accuracy inrepresenting the errors and hence increase the achievable data rate.Representing the error feedback signals more accurately may also causefaster convergence in the output of the precoder, e.g. using the LMSalgorithm, and therefore reduce the training time.

FIG. 7 illustrates another embodiment of a data rate improvement 700during a precoder training time. The data rate improvement 700 is shownfor simulated pilot symbols in a joining line, which may be configuredsubstantially similar to the pilot symbols in FIG. 6. However, in FIG.7, the quantity of bits used to represent the pilot symbols during thetraining time is fixed at about four bits. The data rate improvement 700may be represented by a curve 710. The curve 710 may comprise a datarate value for each of the transmitted pilot symbols during the precodertraining time. The pilot symbols may be transmitted by adjusting theerror vector and hence the accuracy of the error feedback signals. Theerror vector was adjusted by selecting a scaling factor value from thevalues 1, 2, 4, 8, 16, 32, 64, 128, and 256. Additionally, the curve 710was obtained using a LMS step size μ equal to about 0.01. As shown inFIG. 7, the data rate value may increase and converge as the trainingtime increases. The curve 710 may be compared to another curve 720,which may comprise the data rate values in an ideal precoder. The datarate value in the curve 710 may reach about 143 Mbps after about 24seconds, which may be substantially close to the data rate value of thecurve 720.

The curve 710 may also be compared to curves 730 and 740, which maycomprise the data rate values for the transmitted pilot symbols usingconventional quantization and the LMS algorithm. The curve 730 wasobtained using a LMS step size μ equal to about 0.01, and the curve 740was obtained using a smaller LMS step size μ equal to about 0.003. Thedata rate value in the curve 730 may reach about 90 Mbps after about tenseconds, which shows a faster training time in comparison to the curve710. However, the curve 730 may achieve a substantially lower data rateat convergence than the curve 710. In comparison to the curve 730, thedata rate value in the curve 740 may reach about 118 Mbps after about 50seconds, which shows an improvement in achievable data rate at theexpense of additional training time. Thus, the curve 710 showsimprovement in both training time and achievable data rate than thecurves 730 and 740, which may indicate that scaling the error vector toadjust the quantization accuracy may improve the precoder trainingprocess and performance without substantially increasing overhead.

FIG. 8 illustrates an embodiment of an error feedback message 800, whichmay be sent from the CPE to the Exchange. The error feedback message 800may comprise a plurality of error feedback values 810, which maycorrespond to a plurality of tones in a pilot symbol. Each errorfeedback value 810 may comprise a real error component 812 and animaginary error component 814. For example, the error feedback message800 may comprise K real error components 812 (e.g. e_(x)(1,i),e_(x)(2,i), . . . , e_(x)(K,i)) and K imaginary error components 814(e.g. e_(y)(1,i), e_(y)(2,i), . . . , e_(y)(K,i)) for K tones in thepilot symbol, where K is an integer. Additionally, the error feedbackmessage 800 may comprise a number of bits 820 per error component. Thenumber of bits 820 may indicate the quantity of quantization bitsN_(r)(i) that is used to represent the real error component 812 andsimilarly the imaginary component 814 for each tone.

FIG. 9 illustrates another embodiment of an error feedback message 900,which may be sent from the CPE to the Exchange. The error feedbackmessage 900 may comprise a plurality of error feedback values 910, whichmay correspond to a plurality of tones in a pilot symbol. Each errorfeedback value 910 may comprise a real error component 912 and animaginary error component 914. For example, the error feedback message900 may comprise K real error components 912 (e.g. e_(x)(1,i),e_(x)(2,i), . . . , e_(x)(K,i)) and K imaginary error components 914(e.g. e_(y)(1,i), e_(y)(2,i), . . . , e_(y)(K,i)) for K tones in thepilot symbol, where K is an integer. Additionally, the error feedbackmessage 900 may comprise a quantization accuracy 920. The quantizationaccuracy 920 may indicate the quantization accuracy d of the real errorcomponent 912 and similarly the imaginary component 914 for each tone.

FIG. 10 illustrates another embodiment of an error feedback message1000, which may be sent from the CPE to the Exchange. The error feedbackmessage 1000 may comprise a plurality of error feedback values 1010,which may correspond to a plurality of tones in a pilot symbol. Eacherror feedback value 1010 may comprise a real error component 1012 andan imaginary error component 1014. For example, the error feedbackmessage 1000 may comprise K real error components 1012 (e.g. e_(x)(1,i),e_(x)(2,i), . . . , e_(x)(K,i)) and K imaginary error components 1014(e.g. e_(y)(1,i), e_(y)(2,i), . . . , e_(y)(K,i)) for K tones in thepilot symbol, where K is an integer. Additionally, the error feedbackmessage 1000 may comprise a number of bits 1020 per error component anda quantization accuracy 1030. The number of bits 1020 and thequantization accuracy 1030 may indicate the quantity of quantizationbits N_(r)(i) and the quantization accuracy d, respectively, for thereal error component 1012 and similarly the imaginary component 1014 foreach tone.

FIG. 11 illustrates an embodiment of an error feedback signaling method1100, which may be used during a crosstalk precoder initialization ortraining time. The error feedback signaling method 1100 may beestablished between a CPE and an Exchange or a crosstalk precodercoupled to the Exchange. The method 1100 may begin at block 1110, wherea pilot symbol that comprises a plurality of tones may be received. Forexample, the pilot symbol may be transmitted by a VTU-O at the Exchangeand received by a VTU-R at the CPE via a subscriber line. Next, at block1120, the errors in the tones of the received pilot symbol may bemeasured. For example, the VTU-R may measure the error in each tone,which may result from crosstalk in the subscriber line from adjacent orother subscriber lines. At block 1130, the maximum error in the tonesmay be obtained. For example, the maximum error in the plot symbole_(max)(i) may be the maximum error component in the tones, such as froma plurality of real and imaginary error components based on equation(4).

At block 1140, a quantity of bits and/or a quantization accuracy may bedetermined for an error feedback signal that indicates the measurederrors in the tones. For instance, the quantity of bits N_(r)(i) may bedetermined based on a predetermined accuracy d and the maximum errore_(max)(i), e.g. using equation (5). As such, the quantity of bits forthe error feedback signal may be reduced as the errors in the tonesdecrease to reduce the training time of the crosstalk precoder.Alternatively, the quantization accuracy d may be determined based on afixed quantity of bits N_(r)(i) and the maximum error e_(max)(i), e.g.using equation (6). Accordingly, the error vector for the error feedbacksignal may be scaled, e.g. using a scaling factor S_(Q)(i) and equations(7a) and (7b). As such, the quantization accuracy for the error feedbacksignal may be increased to improve system performance by increasing theachievable data rate after training the crosstalk precoder. In otherembodiments, both the quantity of bits and the quantization accuracy maybe adjusted based on the maximum error e_(max)(i) to achieve anacceptable or desired balance (or tradeoff) between the training timeand the achievable data rate.

Next, at bock 1150, the error feedback signal may be transmitted usingthe determined quantity of bits and/or based on the determinedquantization accuracy. For example, the CPE may send an error feedbackmessage to the Exchange, such us the error feedback message 800, theerror feedback message 900, or the error feedback message 1000. In someembodiments, the error feedback message may be configured similar to theR-ERROR_FEEDBACK message in Table 10-4 of the ITU-T standard G.vectorand comprise a field “Quantization Scaling Factor” that indicates thequantization accuracy. At block 1160, the method 1100 may determine if anext pilot symbol has been received. If the condition in block 1160 ismet, the method 1100 may return to block 1120 to measure the errors inthe next pilot symbol and transmit an error feedback signal accordingly.Otherwise, the method 1100 may end.

The components described above may be operated in conjunction with anygeneral-purpose network component, such as a computer or networkcomponent with sufficient processing power, memory resources, andnetwork throughput capability to handle the necessary workload placedupon it. FIG. 12 illustrates a typical, general-purpose networkcomponent 1200 suitable for implementing one or more embodiments of thecomponents disclosed herein. The network component 1200 may include aprocessor 1202 (which may be referred to as a central processor unit orCPU) that is in communication with any memory devices includingsecondary storage 1204, read only memory (ROM) 1206, random accessmemory (RAM) 1208, input/output (I/O) devices 1210, and networkconnectivity devices 1212, or combinations thereof. The processor 1202may be implemented as one or more CPU chips, or may be part of one ormore application specific integrated circuits (ASICs).

The secondary storage 1204 is typically comprised of one or more diskdrives or tape drives and is used for non-volatile storage of data andas an over-flow data storage device if RAM 1208 is not large enough tohold all working data. Secondary storage 1204 may be used to storeprograms that are loaded into RAM 1208 when such programs are selectedfor execution. The ROM 1206 is used to store instructions and perhapsdata that are read during program execution. ROM 1206 is a non-volatilememory device that typically has a small memory capacity relative to thelarger memory capacity of secondary storage 1204. The RAM 1208 is usedto store volatile data and perhaps to store instructions. Access to bothROM 1206 and RAM 1208 is typically faster than to secondary storage1204.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R_(l), and an upperlimit, R_(u), is disclosed, any number falling within the range isspecifically disclosed. In particular, the following numbers within therange are specifically disclosed: R=R_(l)+k*(R_(u)−R_(l)), wherein k isa variable ranging from 1 percent to 100 percent with a 1 percentincrement, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5percent, . . . , 50 percent, 51 percent, 52 percent, . . . , 95 percent,96 percent, 97 percent, 98 percent, 99 percent, or 100 percent.Moreover, any numerical range defined by two R numbers as defined in theabove is also specifically disclosed. Use of the term “optionally” withrespect to any element of a claim means that the element is required, oralternatively, the element is not required, both alternatives beingwithin the scope of the claim. Use of broader terms such as comprises,includes, and having should be understood to provide support fornarrower terms such as consisting of, consisting essentially of, andcomprised substantially of. Accordingly, the scope of protection is notlimited by the description set out above but is defined by the claimsthat follow, that scope including all equivalents of the subject matterof the claims. Each and every claim is incorporated as furtherdisclosure into the specification and the claims are embodiment(s) ofthe present disclosure. The discussion of a reference in the disclosureis not an admission that it is prior art, especially any reference thathas a publication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: a transceiver configuredto: receive a sync symbol comprising a plurality of tones; determine anerror component for each of the plurality of tones to generate aplurality of error components, wherein each error component comprises areal error component and an imaginary error component; and send afeedback error message to train a precoder, wherein the feedback errormessage comprises the plurality of error components and an indicationthat determines at least one of a quantity of bits per error componentand a quantization accuracy per error component, wherein a maximum errorfeedback value of the sync symbol is calculated based at least in parton the real component and the imaginary component of the error componentof each of the plurality of tones, and wherein the quantity of bits pererror component and/or the quantization accuracy per error component aredetermined according to the maximum error feedback value.
 2. Theapparatus of claim 1, wherein the transceiver is a very high bit ratedigital subscriber line (VDSL) transceiver remote unit (VTU-R) that isfurther configured to determine at least one of the quantity of bits pererror component and the quantization accuracy per error component. 3.The apparatus of claim 1, wherein the precoder is a crosstalk precoder,and wherein the error components correspond to a level of crosstalkerror in a digital subscriber line (DSL).
 4. An apparatus comprising: atleast one processor configured to: determine a range of error for aplurality of error components of a pilot signal; determine a quantity ofbits for representing error such that a full error range is preserved, aquantization accuracy such that the full error range is represented by afixed number of feedback bits, or both based on the range of error forthe error components; and transmit an error feedback signal thatcomprises the error components and indicates the quantity of bits, thequantization accuracy, or both, wherein each of the error componentscomprises a real component and an imaginary component, wherein a maximumerror feedback value is calculated based at least in part on the realcomponent and the imaginary component of each of the error components,and wherein the quantity of bits and/or the quantization accuracy aredetermined according to the maximum error feedback value.
 5. Theapparatus of claim 4, wherein the error feedback signal is representedin complex format:E(k,i)=e _(x)(k,i)+j·e _(y)(k,i), where E(k,i) is the error feedbacksignal for the pilot symbol i and a tone k of the pilot symbol,e_(x)(k,i) is the real component of the error feedback signal, ande_(y)(k,i) is the imaginary component of the error feedback signal. 6.The apparatus of claim 5, wherein the maximum error feedback value isobtained from the real component e_(x)(k,i) and the imaginary componente_(y)(k,i) by:${{e_{\max}(i)} = {\max\limits_{k}\left\{ {\max\left\{ {{{e_{x}\left( {k,i} \right)}},{{e_{y}\left( {k,i} \right)}}} \right\}} \right\}}},$where e_(max)(i) is the maximum error in the pilot symbol i and max{ }indicates a function for selecting a maximum sample from a set.
 7. Theapparatus of claim 6, wherein the quantization accuracy is fixed and thequantity of bits is determined by:${{N_{r}(i)} = {\log_{2}\left( \frac{2{e_{\max}(i)}}{d} \right)}},$where N_(r)(i) is the quantity of bits for the pilot symbol i and d isthe quantization accuracy.
 8. The apparatus of claim 6, wherein theerror feedback signal is represented using a quantization format:E _(x)(k,i)=max{−2^(N-1),min{S _(Q)(i)·e _(x)(k,i)·2^(N-1),2^(N-1)−1}},E _(y)(k,i)=max{−2^(N-1),min{S _(Q)(i)·e _(y)(k,i)·2^(N-1),2^(N-1)−1}},where E_(x)(k,i) is the real component of the error feedback signal,E_(y)(k,i) is the imaginary component of the error feedback signal, andS_(Q)(i) is a scaling factor.
 9. The apparatus of claim 4, wherein thequantity of bits and the quantization accuracy for the error componentsdetermine a full range error in the pilot symbol.
 10. A methodcomprising: receiving a sync symbol comprising a plurality of pilottones; determining a plurality of error signals for the pilot tones;quantizing the error signals to generate an error vector comprisingquantized error signals; sending an error feedback message to a digitalsubscriber line (DSL) crosstalk precoder to train the crosstalkprecoder, wherein the error feedback message comprises the error vectorand an indication that determines at least one of a quantity of bits pererror signal and a quantization accuracy per error signal, wherein theerror vector comprises a real component and an imaginary component,wherein a maximum error feedback value is calculated based at least inpart on the real component and the imaginary component of the errorvector, and wherein the quantity of bits per error signal and/or thequantization accuracy per error signal are determined according to themaximum error feedback value.
 11. The method of claim 10, wherein theDSL crosstalk precoder is trained based on the error feedback messageand using a least mean square (LMS) algorithm.